The most commonly used cast materials for abrasive applications are those contained in ASTM A532, "Abrasion-Resistant Cast Irons". Although there are a number of grades within some of the classifications, the alloys are grouped into three main classes as follows:
______________________________________ ASTM Class Comments ______________________________________ Class I These are lower chromium cast irons contain- ing 1 to 11% chromium and 3-7% nickel, com- monly referred to as NI-Hard. Class II These are higher chromium cast irons con- taining 11 to 23% chromium, with the addi- tion of 0.5 to 3.5% molybdenum, commonly re- ferred to as moly alloyed high chromium irons. Class III These are the straight high chromium cast irons containing 23 to 28% chromium. They are commonly referred to as the 25 chrome irons. ______________________________________
Class I alloys (Ni-Hard) containing 3-7% nickel are heat treated to be essentially martensitic (some retained austenite may be present), with chromium and iron carbides. They have a typical Brinell hardness of 500-600. The most common grade is Type D, sometimes called Type 4, containing about 9% chromium.
Class II alloys containing molybdenum also are essentially martensitic after heat treatment, with chromium and iron carbides.
However, Class II alloys can be annealed to reduce the hardness to about 450 Brinell for limited machining.
Class III alloys are essentially martensitic when heat treated, containing chromium and iron carbides. However, in section thicknesses over about two inches, these cast irons are partially or wholly pearlitic. Although this increases the impact resistance, the wear resistance is reduced. As with the Class II alloys, these 25 chromium irons can be annealed for machining. In the hardened condition, they have a Brinell hardness of about 550 to 600.
In general, it should be stressed that these three classes of materials, even with those that can be annealed, machining is next to impossible and welding should never be allowed. Also, although there is a trade off between carbon and chromium (as the carbon is reduced, more chromium is available for corrosion resistance), in general the corrosion resistance is not very good, particularly at low pH values. Unfortunately, some of the more recent applications for slurry pumps such as used in scrubbers, contain liquids having low pH values (less than 4).
It should be noted that other materials have been used for abrasive applications where corrosion is a problem, with one of the more popular being the duplex stainless steel alloy CD-4MCu which can behardened to about 300-325 Brinell with an aging treatmet. Although expensive, the cobalt base Stellite alloys have excellent abrasion resistance.
Based on the previous comments, the selection of a mttallic abrasion resistant alloy depends upon the end use, where one must consider not only the section size, but the corrosiveness of the liquid. Since the abrasion resistant cast irons do not possess passive films in the sense of the austenitic stainless steels, they are not very good under acidic conditions. However, if one attempts to use an which does have a fairly stable passive film, the particulates may prevent this film from forming.
It should be noted that many foundries have their own modifications of these three classes of abrasion resistant alloys and often they will select one of their own "alloys" for a particular application. However, from a metallurgical standpoint, the abrasion resistant cast irons can be quite complex containing numerous types of carbides having various morphological characteristics as well as a matrix which can contain martensite, austenite or even the transformation products, pearlite and bainite. Although subtle differences can produce dfffering abrasion resistance, the gains are relatively insignificant.
For pump components, such as impellers and casings, the Type III alloy (25% chromium), is the most widely used. However, based on the preceding discussion, it is very difficult to manufacture, is very brittle and has poor corrosion resistance, particularly at low pH values.
This invention describes a new type of abrasion resistant alloy having superior abrasion resistance as well as superior corrosion resistance compared to the classical ASTM A532 type alloys.
Accordingly, it is an object of this invention to provide an abrasion-corrosion resistant casting alloy comprising the following range of composition:
______________________________________ C Mn Si Cr Cu N V Ti Mo ______________________________________ % min. 0.1 3.0 1.0 26.0 1.0 0.3 0.5 0.5 1.0 % max. 0.5 7.0 5.0 34.0 2.0 0.7 1.5 1.5 3.0 ______________________________________
with the balance being Fe; and substantially the following anticipated heat treatment:
1. Solution treat at 2050.degree. F. (1121.degree. C.) to 2250.degree. (1232.degree. C.) for 1 hour per inch of thickness followed by a suitable quench, for example oil, or accelerated air cool.
2. Heat to 1600.degree. F. (871.degree. C.) to 1800.degree. F. (982.degree. C.) for 6 hours.
3. Furnace cool from the temperature in step 2, at a maximum rate of 50.degree. F./hour to the range of 1100.degree. F. (593.degree. C.) to 1200.degree. F., (648.degree. C.) followed by cooling in still air.